A rigid printed circuit board (RPCB) used in the related art, which is a laminate of epoxy or phenol-based resin and a conductive metal layer of copper and the like, has a problem in that it does not meet required characteristics of current electronic devices, which are being manufactured as light, slim, short, and small devices. A flexible printed circuit board appearing as an alternative to the rigid printed circuit board has been widely used for a mobile phone, a notebook, a liquid crystal display (LCD) and the like due to easiness of lightweight and slimness and its own inherent flexibility.
A metal-clad laminate used to manufacture the flexible printed circuit board is largely sorted into a three-layer type and a two-layer type. The three-layer type is a form that stacks an insulating film such as polyimide and the like and a conductive metal layer by using an adhesive and the two-layer type is a form where the metal layer is directly bonded on the insulating film without using an adhesive.
However, since the existing three-layer type using the adhesive has the adhesive layer, it has disadvantages in that heat-resistance and flame retardance are degraded and the dimension is greatly changed during an etching process and a heat treatment process. Recently, a study on the metal-clad laminate in the two-layer type that directly stacks the insulating film and the metal layer has been actively progressed.
Generally, the metal-clad laminate in the two-layer type has a problem in that the curl, warp, twist and the like easily occur after and before the etching process due to the thermal expansion difference between the insulating film and the metal layer. As a method for solving the above problem, U.S. Pat. No. 4,847,353, etc., proposed a method of using low thermal expansible polyimide having a specific structure; however, this method cannot be a fundamental solution because adhesive strength is low as well as a conductor is excessively curled inwardly during a process of forming a circuit by etching the conductor.